I hope you've tried "Live Help" already and the matter is resolved(?), but if not:

On Mac - You should be able to pick a Midi instrument for a track that already has a Realtrack by ctrl-clicking the track button and select the "Select Custom Midi Style for this track" from the pull-down menu.
Alternatively, go into the Realtrack Picker for your desired track and click on the "none" button, which should remove the RT from that track. It may be that you need to do this first anyway for your version of the program, I can't remember. I also can't remember if Supertracks had been incorporated back then, in which case you have that option too.

Just because you can, doesn't mean you should!
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BBox 2022 Audiophile, Mac Pro Intel, OSX 10.6.8, 800x600 (TV VGA)